The present invention relates to an improved notch filter circuit and more particularly, to a notch filter constructed from a novel helical transmission line inductance and coaxial capacitor. The notch filter has application in a variety of circuits, including tuners for use in satellite receivers.
A tuner for a satellite receiver has to provide several signal processing functions. Among these are to receive an RF signal (e.g., from an antenna) providing reception of a plurality (e.g., 16) channels, with each channel in the -65 dBm up to the -20 dBm level range. The RF signal must then be down-converted to an appropriate intermediate frequency ("IF"), and provided to a demodulator at a constant IF signal level over the entire RF signal range.
In addition to the above, a tuner for a satellite receiver that operates in the C-band (3.7 to 4.2 gigahertz ("GHz")) requires a terrestrial interference ("TI") filter to reject the interference caused by terrestrial microwave communications. Such microwave communications occur in the same frequency range (i.e., 3.7 to 4.2 GHz) as satellite television.
In order to be useful, a tuner for use in a satellite receiver must perform the above functions with minimum signal quality degradation and at minimum cost. To preserve the signal quality, a TI filter used in a C-band satellite receiver tuner should operate with a fairly sharp notch, typically on the order of 4 MHz maximum bandwidth at the 3 dB point, with a 15 dB notch depth. In order to ensure that this specification is met for all conditions, including component tolerance, gain/loss variations, temperature variations, and the like, the TI filter should be designed to provide, at nominal conditions, a 3 MHz bandwidth at 3 dB, with a 15 dB notch depth.
The specifications set forth above pose difficulty in the design of a suitable TI filter for use in a satellite receiver tuner. The relatively narrow bandwidth of the notch (i.e., 3 MHz at 3 dB) suggests the use of a relatively low IF frequency. The reason for this is that in order to operate at a higher IF, the reactive elements in the filter must have a higher "quality factor" Q. As is well known in the art, Q is a dimensionless parameter, calculated by dividing the product of a filter's resonant frequency and inductance by the resistance of the filter.
The use of a low IF frequency in a satellite tuner has three drawbacks. First, if the IF is lower than about 250 MHz, an image problem arises in the IF conversion process. In order to resolve this problem, additional circuitry such as a tracking RF filter is necessary. Second, with a lower IF it is more difficult to achieve linear FM ("frequency modulation") demodulation, particularly in quadrature demodulators. Linear FM demodulation is also difficult with a low IF because it is more difficult to keep the group delay low in IF filters at lower frequencies. Third, high quality FM demodulator integrated circuit chips are currently available only at frequencies above 300 MHz.
For these reasons, it is preferable to provide a satellite receiver tuner with an IF of above 300 MHz. At such frequencies, a TI filter will require an approximate Q of 1,000, and a stability of approximately 20 ppm/ .degree.C. This is difficult to achieve, even with costly surface acoustic wave ("SAW") devices.
It would be advantageous to provide a TI filter that operates above 300 MHz, to enable the construction of a satellite receiver tuner or the like that meets the criteria set forth above. It would be further advantageous to provide such a TI filter that is economical, reliable, and easy to construct.
The present invention provides such a TI filter, that is implemented in helical transmission line techniques.